This research is to determine the role of leukocytes in thrombotic disease. It has already been determined that leukocytes can develop thromboplastic activity, that they are specifically located in thrombi adjacent to the fibrin, and that they are required for endotoxin induced intravascular coagulation and the generalized Shwartzman reaction. Experimental thrombi will be studied to determine the effect of agranulocytosis on the thrombotic process. Leukocyte thromboplastic activity will be studied to determine the materials which stimulate it, the factors necessary for its generation and to define its subcellular and chemical nature.